With the advent of the communications era, one of the major markets for optical devices is portable electronics equipment, such as cellular telephones, pagers, two-way radios, data banks, computers and the like. Generally, it is desirable that the optical source devices in this type of equipment are compact with low power, inexpensive and include high quality optics with large angular magnification. However, large angular magnification requires a small focal length, as can be seen from the relationship for angular magnifying power (MP) set forth below. Generally, in the discussion below the optical system is treated as a single lens for convenience in describing the relationships. EQU MP=V(b+f)/[f(b+e)]
where:
V is a constant, the distance of distinct vision (approximately 10" or 254 mm); PA1 b is the distance from the lens to the virtual image; PA1 e is the distance from the lens to the eye; and PA1 f is the effective focal length.
A small effective focal length in turn requires that the optical system have a small F/No., which translates into a fast system in the applications being described. Generally, a fast system is defined as a system with an F/No. less than or equal to approximately 2. As is known in the art, the F/No. is determined by the ratio of the focal length to the diameter of the lens.
Thus, as the focal length is reduced and the system is made smaller, it is inherent that a larger area of the lens, or lens system, is used. This use of a larger area of the lens system results in greater aberration in images transmitted through the system and a need for greater aberration correction. As is well understood by those skilled in the art, greater aberration correction means more elements in the system, which in turn means larger size and greater cost. Therefore, a major problem that is inherent in any attempts to produce compact, inexpensive, high quality optics with large angular magnification is aberration correction.